Field of the Invention
The present invention relates to a laminate structure suitable for use as electrical insulation in such things as motors, generators, and inverters.
Description of the Related Art
To those skilled in the art, the term “mica paper” refers to sheet made with high concentrations of the inorganic mineral mica, generally in an amount that is at least 90 weight percent or greater, with the remainder being a binder that provides some mechanical integrity to paper. However, the resulting mica paper is not a mechanically strong paper.
U.S. Pat. Nos. 6,991,845 and 7,399,379 to Levit et al. discloses a sheet structure for electrical insulation or flame resistance comprising a barrier ply having a mica-rich face and a mica-poor face and a reinforcing ply containing a saturable backing layer attached to the mica-poor face of the barrier ply.
U.S. Pat. No. 6,991,845 to Levit et al. and U.S. Pat. No. 6,312,561 to Forsten et al. disclose an aramid-mica blended paper made from a homogenous blend of m-aramid fiber, m-aramid fibrids, and mica. Levit et al. further teaches that this “aramid-mica paper” has superior mechanical properties when compared to “mica paper” and can be used without any backing reinforcement.
In some instances, the use of “calcined” mica in sheet structures utilized in the high voltage electrical insulation market is desirable because the calcined mica has improved dieletric properties over “uncalcined” mica. Calcination provides smaller-sized mica platelets when compared to natural or uncalcinated mica flakes, which is believed to improve the dielectric performance of the mica in the sheet. Unfortunately these smaller-sized mica platelets also form a less porous layer, meaning the layer can be difficult to wet out with impregnating matrix resins.
Therefore what is needed are improved methods to make laminate structures containing calcined mica having improved porosity.